chore: add local NuGet source via NOSCORE_LOCAL_PACKAGES env var (#761)

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: erwan-joly

NuGet.config

@@ -1,6 +1,7 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <configuration>
     <packageSources>
+        <add key="local" value="%NOSCORE_LOCAL_PACKAGES%" />
         <add key="nuget.org" value="https://api.nuget.org/v3/index.json" />
     </packageSources>
 </configuration>